WO2003095947A1

WO2003095947A1 - Hot wire mass flow measurement device for a high temperature gas

Info

Publication number: WO2003095947A1
Application number: PCT/IB2003/001825
Authority: WO
Inventors: Chris Baerts; Chris Ganseman; Patrick Van De Voorde; Joseph R. Griffin
Original assignee: Melexis Nv
Priority date: 2002-05-10
Filing date: 2003-05-09
Publication date: 2003-11-20
Also published as: GB0210657D0; AU2003230072A8; WO2003095947A8; AU2003230072A1

Abstract

An apparatus for measuring the mass flow of a high temperature gas stream 104 comprises a heating element 101 maintained at a predetermined temperature by a control means 202 which controls the flow of current to the heating element from a current source 201, a mass flow rate calculating means 204 and a temperature measurement means 103 to measure the temperature of gas in the hot gas stream 104. The predetermined temperature of the element is greater than the temperature of the gas in the hot gas stream 104 and is high enough to degrade any deposits formed on the surface of the heating element 101. The mass flow rate is determined by mass flow calculation means 204 from the measured gas stream temperature and the amount of energy dissipated in said element 101 to maintain said predetermined temperature.

Description

HOT WIRE MASS FLOW MEASUREMENT DEVICE FOR A HIGH TEMPERATURE GAS

The present invention relates to apparatus for measuring the mass

flow of a high temperature gas stream particularly but not exclusively for

such a gas stream in an exhaust system of a modern internal combustion

engine.

A known arrangement for measuring gas mass flow rate utilises a

measurement of the cooling effect of the gas on a heated element as it

flows over the element. Arrangements of this type are well known in the

field of internal combustion engines and are utilised in the ambient air

intake system.

However, such systems are not usually suitable for use with gases

such as are found in automotive engine exhaust systems, which gases are

hot, polluted and corrosive. In particular, pollutants in the exhaust gases

can be deposited as a layer onto the surface of the heated element. This

pollutant layer will affect the heat transfer properties between the heated

element and the gas and will therefore reduce the accuracy of

measurements of cooling effect made.

It is therefore an object of the present invention to provide an

arrangement for measuring gas mass flow rate that avoids these inherent

difficulties with the known arrangements.

Thus and in accordance with the present invention therefore there

is provided an apparatus for measuring the mass flow of a high temperature gas stream, said apparatus comprising an electrical heating

element supplied with current from a current source, control means to

control the flow of current through the element from the source,

temperature measurement means to measure the temperature of gas in

the hot gas stream and mass flow rate calculation means to calculate the

mass flow rate wherein the flow of current through the element is

controlled by said control means to maintain said element at a

predetermined temperature, the predetermined temperature being greater

than the temperature of the gas in the hot gas stream and sufficient to

degrade deposits on the surface of the heating element, and a value for

said hot gas mass flow rate is determined by said calculation means from

the measured gas temperature and the amount of energy dissipated in

said element to maintain said predetermined temperature.

With this arrangement it is possible to measure gas mass flow rate

in a high temperature gas stream more efficiently and accurately and to

minimise the effects of pollutants in the gas on the measurement.

Preferably, the heating element is a resistor with a positive

temperature coefficient.

The apparatus may be adapted to measure the mass flow rate of an

exhaust gas stream of an engine, preferably an internal combustion

engine. In order to achieve this the heating element and the temperature

measurement means are positioned such that they project into the

exhaust gas stream. Preferably the engine has an engine control unit and most preferably the engine control unit provides the current source,

control means and mass flow calculation means for the apparatus.

Alternatively, an independent electronic control unit may provide these

functions.

In some embodiments the heating element may be switched

between being maintained at a first predetermined temperature and being

maintained at a second predetermined temperature, in particular wherein

the first predetermined temperature is higher than the gas temperature

and is sufficient to degrade deposits on the surface of the heating

element and the second predetermined temperature is higher than the

temperature of the gas stream but is not high enough to degrade deposits

on the surface of the heating element. This allows the heating element to

operate at the lower second predetermined temperature at least part of

the time making the apparatus more reliable and to operate at the first

higher predetermined temperature in order to degrade deposits of

pollutants which form on the surface of the heating element. In typical

operation the heating element is maintained at the first predetermined

temperature for less time than it is maintained at the second

predetermined temperature.

The times at which the heating element is switched between the

first predetermined temperature and the second predetermined

temperature are determined according to any suitable function of time. In

a preferred embodiment, the heating element is maintained at the first predetermined temperature for a time interval after being switched on and

then maintained at the second predetermined temperature thereafter. In a

particularly preferred embodiment the timing of the switching between the

first predetermined temperature and the second predetermined

temperature is determined from an integration of engine effort coefficient

calculated by the engine control unit.

The accuracy of the measurement of mass flow rate may be

affected by such parameters as the specific heat of the gas, the gas

pressure, including static gas pressure, engine speed, engine load, lambda

value and, where the arrangement is used in an internal combustion

engine, pressure pulsations caused by normal operation of the engine.

Accordingly correction factors may be used to adjust the value obtained

from a simple calculation of gas mass flow rate to account for these

parameters. Preferably storage means are provided in which suitable

correction factors may be stored.

The invention will now be described by way of example only and

with reference to the accompanying drawings, the single Figure of which

shows, in schematic form, one embodiment of arrangement for measuring

gas mass flow rate in accordance with the present invention.

Referring now the drawing, there is shown an arrangement for

measuring the mass flow rate of gas in a hot gas stream.

The arrangement comprises a heater element 101 mounted on an

exhaust system of an internal combustion engine. The element 101 projects internally of the exhaust system and extends into the exhaust

gas stream.

The element 101 is maintained at a constant temperature by

passing an electric current through it from a current source 201 . The

current source 201 in the embodiment shown is provided in an electronic

control unit 200 (ECU). The heater element 101 is preferably a positive

temperature coefficient resistor. The current passed through the

temperature coefficient resistor from the current source 201 is adjusted

by a control device 202 to maintain the resistance of the positive

coefficient resistor at a predetermined target value representing a target

temperature for the positive temperature coefficient resistor- The

instantaneous energy dissipated in the positive temperature coefficient

resistor can be determined from the instantaneous value of the current

and the target value of the resistance of the temperature coefficient

resistor and this represents the cooling effect of the air flow on the

heated temperature coefficient resistor. The temperature of the gas in the

gas stream is measured by any suitable temperature sensor 103 also

monitored in the exhaust gas stream. The temperature sensor 103 is

connected via a interface 203 to a mass flow rate calculation device 204

which is, in the embodiment shown, also provided in the ECU. The heater

element 101 is also connected to the calculation device via current source

201 and control means 202. The cooling effect of the gas flow is a function of the mass flow

rate, the area of the positive temperature coefficient resistor in contact

with the gas flow and the difference between the temperature of the gas

in the hot stream and the temperature of the positive temperature

coefficient resistor. Therefore by appropriate calculation, the gas mass

flow rate can be calculated from these values in the calculation device.

In order to obtain greater accuracy in measurement of the mass

flow rate, the temperature of the gas is preferably measured at a point

where the effects of the heating of the positive temperature coefficient

resistor on the temperature of the gas are minimal and at an acceptable

level. Furthermore, it will be appreciated that the target temperature of

the positive temperature coefficient resistor must be higher than the

temperature of the gas in the hot stream by an amount large enough for

the cooling effect of the gas stream to be measured.

The area of the positive temperature coefficient resistor in contact

with the gas can be reduced by deposits of pollutant on the surface of the

heater. To reduce or eliminate the deposition of the pollutant onto the

surface of the positive temperature coefficient resistor, the positive

temperature coefficient resistor, is in the present invention, operated at a

temperature above that which pollutants are burnt off (the pollutant burn

off temperature). This will clearly avoid the problems associated with the

known arrangements where the pollutant deposits on the surface on the

heater effecting measurement. However, operation at high temperature can have a deleterious effect on the reliability of the temperature

coefficient resistor itself and therefore, it is advantageous for the target

temperature to be adjusted to be above the pollutant burn off temperature

for only a part of the time and at a lower temperature for the remainder of

the operating time. In order to achieve this, the arrangement of the

invention may switch between these two temperatures as a simple

function of time. Alternatively, where the arrangement is incorporated in

an internal combustion engine, there may be one high temperature period

during which pollutants are burnt off the temperature resistor at every

engine start up. In a particularly preferred arrangement, the timing of

switching to the high temperature is determined from a calculated

"integration of engine effort" coefficient. This calculation is known in the

art and is based on time, engine speed, gear selection, throttle position

and other such parameters and is designed to predict the likelihood of

pollutant formation in the exhaust gases.

The deposition of pollutant on the temperature sensor which

measures the temperature of the gas in the hot gas stream is of lesser

importance since such a deposition does not effect accuracy but only the

response time of the sensor to temperature changes.

However, there are other parameters that can affect the accuracy

of the measurement of mass gas flow by the arrangement of the

invention. Such parameters include the specific heat of the gas, the static

pressure of the gas and, where the arrangement is used in an internal combustion engine, pressure pulsations caused by normal operation of the

engine.

In these circumstances, it has been realised that correction factors

can be used to adjust the value obtained from a simple calculation of gas

mass flow rate to account for these parameters.

For example, the variation in specific heat between a 30: 1 air/fuel

ratio diesel exhaust gas and stoichiometric diesel exhaust gas is

approximately 5%. This can be compensated for by a factor derived from

a measurement of excess oxygen in the hot gas. In a preferred

embodiment therefore, the arrangement of the invention utilises a

compensation table which gives a correction factor for various values of

excess oxygen which is to be applied to be calculated gas mass flow rate.

Furthermore, the variation in static pressure of the hot gas can

affect the density of the gas and therefore the velocity at given flow

rates. To a lesser extent, it also affects the specific heat of the gas. The

arrangement of the invention, when used in a diesel engine, can be

compensated for by utilisation of a stored correction factor derived from

pressure measurements.

In the exhaust systems of internal combustion engines there is

often a pulsating variation in gas pressure brought about by the opening

and closing of the exhaust valves in the engine. These pressure

pulsations are a function of engine speed and load and their magnitude at

various speed and load points is repeatable. A compensation factor can be determined for a range of speed and load values and can be stored in a

two-dimensional array in the arrangement of the invention.

It is preferred that mass flow sensors for use in the arrangement of

the invention are tested and calibrated in a test rig using test gases and

test gas temperatures before use. The mass flow sensors are calibrated

on the test rig and test results recorded. Parameters are calculated from

the measured test results that relate to the performance of the mass flow

sensor under the test conditions and are known by experimentation,

empirical knowledge or calculation to be capable of defining the

performance of one sensor relative to a standard sensor.

These parameters are then recorded. When the temperature sensor

is installed into an operating exhaust system, these parameters are

entered into a controlling engine control unit which assists in optimising

the performance of the temperature sensor. The parameter can be stored

in the engine control system and can be distributed with the temperature

sensor or can be acceptable at data points and made available via

communication means. The parameters may also be stored within non¬

volatile memory means which may form a part of the temperature sensor

itself.

It is of course to be understood that the invention is not intended

to be restricted to the details of the above embodiments which are

described by way of example only.

Claims

1 . An apparatus for measuring the mass flow of a high temperature

gas stream, said apparatus comprising an electrical heating element

supplied with current from a current source, control means to

control the flow of current through the element from the source,

temperature measurement means to measure the temperature of

gas in the hot gas stream and mass flow rate calculation means to

calculate the mass flow rate wherein the flow of current through

the element is controlled by said control means to maintain said

element at a predetermined temperature, the predetermined

temperature being greater than the temperature of the gas in the

hot gas stream and sufficient to degrade deposits on the surface of

the heating element, and a value for said hot gas mass flow rate is

determined by said calculation means from the measured gas

temperature and the amount of energy dissipated in said element to

maintain said predetermined temperature.

2. An apparatus as claimed in claim 1 wherein the heating element is

a resistor with a positive temperature coefficient.

3. An apparatus as claimed in any preceding claim wherein the

apparatus is adapted to measure the mass flow rate of an exhaust

gas stream of an engine.

4. An apparatus as claimed in any preceding claim wherein the engine

is an internal combustion engine.

5. An apparatus as claimed in claim 3 or claim 4 wherein the engine

has an engine control unit.

6. An apparatus as claimed in claim 5 wherein the current source is

the engine control unit.

7. An apparatus as claimed in claim 5 or claim 6 wherein the control

means is the engine control unit.

8. An apparatus as claimed in any one of claims 5 to 7 wherein the

mass flow rate calculation means is the engine control unit.

9. An apparatus as claimed in any preceding claim wherein the

heating element is switched between being maintained at a first

predetermined temperature and being maintained at a second

predetermined temperature.

10. An apparatus as claimed in claim 9 wherein the second

predetermined temperature is higher than the temperature of the

gas stream but is not high enough to degrade deposits on the

surface of the heating element.

1 1 . An apparatus as claimed in claim 9 or claim 10 wherein the times

at which the heating element is switched between the first

predetermined temperature and the second predetermined

temperature are determined according to a function of time.

12. An apparatus as claimed in any one of claims 9 to 1 1 wherein the

heating element is maintained at the first predetermined temperature for less time than it is maintained at the second

predetermined temperature.

13. An apparatus as claimed in any one of claims 9 to 12 wherein the

heating element is maintained at the first predetermined

temperature for a time interval after being switched on and then

maintained at the second predetermined temperature thereafter.

14. An apparatus as claimed in any one of claims 9 to 13 when

dependant upon any one of claims 3 to 8 wherein the timing of the

switching between the first predetermined temperature and the

second predetermined temperature is determined from a calculated

integration of engine effort coefficient.

1 5. An apparatus as claimed in claim 14 when dependant upon any one

of claims 5 to 8 wherein the integration of engine effort coefficient

is calculated by the engine control unit.

16. An apparatus as claimed in any one of claims 3 to 1 5 wherein the

heating element and the temperature sensing element each project

separately into the exhaust gas stream of said engine.

17. An apparatus as claimed in any preceding claim wherein optimised

operating parameters are determined by calibration with a test rig

using test gases and test temperatures.

1 8. An apparatus as claimed in any preceding claim wherein storage

means are provided for storing correction factors for calculating a

corrected mass flow rate.